SUMMARY/ABSTRACT The standard approach of investigating neurodegenerative disorders separately overlooks any fundamental pathological processes that may contribute to multiple disease states. The proposed supplemental research, an administrative supplement to our NIH/NIEHS RO1 ES016931 ?Gene Neurotoxicant Interactions in Huntington Disease?, will bridge this gap in the literature in relation to Alzheimer's Disease (AD) and Huntington's Disease (HD), and demonstrate the extent to which altered manganese (Mn) homeostasis may underlie deficits in gluta- mate clearance and excitotoxicity in both diseases. Thus, we are responsive to NOT-AG-18-008 by adding research relevant to AD to a grant that is not currently focused on AD. There is evidence for altered man- ganese levels, a strong oxidative stress component, and shared risk for seizures in both AD and HD. There is also strong evidence for dysfunction of the glutamate uptake-ascorbate release mechanism within astrocytes in striatum in a mouse model of HD (R6/2), but this has not been studied in other brain areas, or in AD. Our central hypothesis is that AD and HD share a common pathophysiological pathway of glutamate excitotoxicity that is compounded by Mn dyshomeostasis and altered synaptic ascorbate release. Our long-term objectives are to isolate the role of Mn in altered glutamate clearance in AD and HD. In Specific Aim 1 we will determine the extent to which Mn alters the critical glutamate uptake-ascorbate release exchange mechanism in both HD and AD models. The YAC128 HD mouse model and APP/PSEN1 AD model will used to isolate brain slices containing specific brain regions. We will test regional and genotype differences in excitatory-stimulated ascorbate release in ex vivo slices and in vivo using fast scan cyclic voltammetry at baseline, or following exposure to Mn. In Specific Aim 2 we will assess the extent to which AD-related astrocytes have altered sensitivity to Mn. We will directly measure glutamate clearance in cultured astrocytes from YAC128 and APP/PSEN1 mice, under altered Mn conditions. We will further assess neuronal protection in neuron-astrocyte co-cultures. Finally, we will confirm and extend these findings by establishing the extent to which glutamate uptake differs in astrocytes derived from hiPSCs from AD and HD populations. Together these data will confirm the extent of shared Mn dysregulation between the two disease models, and specifically address the role of astrocytes in glutamate clearance. Under- standing these mechanisms will highlight an under-studied role for altered Mn handling in both diseases and provide a new target for therapeutic interventions. This work will provide clinically-measurable biomarkers for enhanced risk, and potential routes for therapeutic intervention. Isolation of regional differences between the two disease states will also reveal vital information about disease-specific susceptibility in certain cell types.